The goal of this proposal is to characterize local synaptic interactions among functionally identified pyramidal cells in the visual cortex. Specific populations of pyramidal cells will be retrogradely labeled with fluorescent markers injected at their long-range axonal targets. By recording simultaneously from multiple labeled neurons in brain slices, we will characterize local synaptic interactions among identified subtypes of pyramidal cells. We will address three specific questions: Aim 1, do pyramidal neurons that project to a common subcortical target selectively connect to each other? Aim 2, does the spatial extent of local connectivity differ among different types of pyramidal cells? Aim 3, do synapses exhibit similar forms of short-term plasticity when grouped as a homogeneous class of connections between functionally defined synaptic partners? The results of these experiments will test (a) whether pyramidal cells connect non-selectively to other neurons or (b) whether each class of pyramidal neuron targets specific partners, forming local excitatory networks within the cortex. These studies will allow us to analyze mouse models of diseases like epilepsy and autism thought to result from aberrant connectivity among neocortical neurons. [unreadable] [unreadable]